Would a massive shade between Earth and the sun help slow climate change?

The concept involves an asteroid counterweight and would be tremendously expensive to pull off.
A NASA image of the sun with a bright solar flare in the upper left.
A solar flare flashes at the edge of the sun, captured by NASA's Solar Dynamics Observatory in 2012. NASA/GSFC/SDO

Some of the most exotic solutions to climate change are the various forms of geoengineering. Such proposals aim to reduce global warming by shrinking the amount of solar radiation that reaches Earth’s surface—by, say, injecting large amounts of sulfur dioxide or dust into the air to mimic the cooling effect of large volcanic eruptions. Or building catapults to launch lunar dust into orbit around Earth and intercept the sun’s rays in the space near our planet. 

But University of Hawaii cosmologist István Szapudi has an even more far-out idea: place a 372,000-mile-wide sun shade tethered to a captured asteroid between Earth and the sun to reduce the amount of solar radiation reaching our planet by 1.7 percent. His analysis is agnostic to the shade’s shape, though he imagines it could be a circular shade made of triangular segments, able to open or close like flower petals to allow variable amounts of sunlight through. 

“It’s not going to cast a sharp shadow,” Szapudi says. ”Maybe with a telescope you could notice that there is something in front of the sun. But other than that, it would just be that people would notice that the weather is a little bit better.”

He readily admits that this concept would require millions of dollars investment in just preliminary engineering studies to see if it is really possible. “Of course, it’s unrealistic to actually do this, so hopefully, we will slowly give up fossil fuels,” Szapudi says, citing a much more mainstream goal to curb a source of climate change. “But that’s a very long-term process.”

An illustration of a triangle-shaped sun shade.
A concept illustration of a sun shade bound to an asteroid. Istvan Szapudi/Institute for Astronomy

In the meantime, he suggests, maybe the world can consider alternatives to help mitigate the change in climate that occurs from the carbon already in Earth’s atmosphere today. 

Szapudi’s proposal, as described in a paper published on July 31 in the Proceedings of the National Academy of Sciences, would place this massive sun shade at the Sun-Earth Lagrange Point 1, or L1. This is a region of space about 932,000 miles toward the sun from Earth where the gravity of both bodies cancels out, allowing a spacecraft orbiting L1 to maintain a constant position relative to the sun and Earth with minimal maneuvering. The James Webb Space Telescope makes use of the same phenomena at L2, the L1 point’s counterpart 932,000 miles away from Earth in the direction of the outer solar system. 

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Szapudi is not the first to suggest placing a sun shade at L1, but previous proposals ran into problems. Namely, a large sun shade will also act like a solar sail, catching solar radiation that will push the structure out of position at L1. Previous proposals got around this by making the sun shade extremely massive, on the order of 350 million tons, perhaps of metal or asteroid stuff—an utterly unrealistic amount of mass even for a proposal that’s already this far out. 

Szapudi instead proposes connecting it to an asteroid counterweight by tethers up to 1.9 million miles long. Since the sun’s gravity is more potent the further away from L1 and closer to the star you go, the tug of solar gravity on the asteroid will counterbalance the radiation pressure on the sun shade, allowing it to stay in place. 

With such a configuration, Szapudi estimated the shade itself might weigh only 35,000 tons. “That’s something that SpaceX could put up in space” using its current rockets, he says, though it’d take a lot of time and effort. A sun shade could be made even lighter, Szapudi suggests, if made from something like graphene, an extremely light and strong material consisting of atom-thick sheets of carbon atoms arranged in a hexagonal lattice pattern. 

Astronomers would have to identify a suitable near-Earth asteroid for the counterweight through something like the University of Hawaii’s Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), Szapudi says. But once they did, the sun shade could be tethered to the asteroid in its existing orbit and used as a solar sail to divert the space rock toward the L1 point. 

Engineering-wise, the whole idea is extremely speculative, Szapudi emphasizes, relying on technology that is not yet developed, such as materials strong and light enough to serve as the tethers. 

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But it’s also not clear if geoengineering of this sort would actually help mitigate the effects of climate change, or do so without introducing other, unpredictable and negative consequences, according to Rutgers University climatologist Alan Robock. Robock leads the Rutgers Geoengineering Model Intercomparison Project, which uses climate change models to predict the effects of geoengineering interventions.

“What if you start doing it and you say, ‘OK, we figured out that 90 percent of the world is going to be better off, but 10 percent is going to be worse off,” Robock says. “But we don’t know which 10 percent because of randomness in the climate system.”

And some effects are well understood, likely, and not good, he adds. 

“For example, you’d get drought in Africa and Asia, because the summer monsoon is driven by the temperature difference between the land and the ocean in the summer,” Robock says. “If you block out the sun, the land would cool more than the ocean. And so that temperature difference would go down. In the summer monsoon precipitation would be reduced.” 

And if something went wrong with the sun shield, and it stopped blocking the sun suddenly, Earth would warm back up much more rapidly than humans have ever experienced r. 

“That’s called the termination problem,” Robock says, and it’s something that dogs all geoengineering proposals. 

And then there’s also the very human problem of cooperating on what is essentially a species-wide project: building and tuning a sun shade. How do humans agree on how much sun to block, or as Robock puts it, how does the world agree on where to set the planetary thermostat? “Countries like Canada and Russia wouldn’t mind it being a little bit warmer,” he says. “In fact, we’ve calculated their agriculture would improve, but countries in the tropics would want it cooler because sea levels are going up, they’re already drowning.”

Ultimately Robock sees geoengineering projects as potential distractions from reducing emissions today. The best solution to climate change, Robock says—and Szapudi agrees—is to leave fossil fuels in the ground. 

 But Szapudi sees his proposal as a project to help mitigate the lasting effects of emissions that have already taken place. It could be an insurance policy to help turn off the worst effects of global warming that are already baked into the climate—but it only works if we start such a long term research project now. 

As an insurance policy, though, it’d have one expensive premium. “If technology develops the way I hope it would, maybe this is a trillion-dollar project,” Szapudi says. “You would need at least an army of engineers, probably tens of millions of dollars just to explore the concept to enough detail.”